High resolution measurement of light frequency or wavelength typically makes use of comparing photon wavelength to another object. One method is to compare the wavelength of light to a physical length, such as the cavity length of a Fabry Perot interferometer. Another method is to compare photon wavelength to the wavelength of other photons, called heterodyne detection. A third method is to compare the energy of a photon, which is proportional to frequency, to an absorption line. A new method is introduced here that measures photon wavelength by an absorption line that relies upon dispersion, which accompanies absorption, of an absorption line.
Optical faraday filters are in widespread use, however optical filter pass band and optical spectrometer range of operation must overlap if they are used together. New apparatus of distinguishing frequency and removing background light utilizing filters, spectrometers and combinations of filters and spectrometers are presented. Applications of these inventions are also presented.